The present invention relates to an oxygen separator and method of separating oxygen that uses oxygen-selective, ion conducting ceramic membranes. More particularly, the present invention relates to such an oxygen separator in which such oxygen-selective ceramic membranes are located within a duct that is either directly connected to an exhaust of a gas turbine or connected to a burner of the gas turbine to receive air heated by combustion of a fuel.
Separation of oxygen from heated, elevated pressure air streams produced by gas turbines can readily be accomplished by oxygen-selective, ion conducting ceramic membranes because gas turbines produce more high temperature air than is required to support combustion within the turbine. In fact, there is a sufficient excess of high temperature air to allow for significant quantities of oxygen to be extracted as a by-product.
There are a number of references in the prior art that disclose integrations of gas turbines with oxygen separators that employ oxygen-selective, ion conducing ceramic membranes (hereinafter referred to in the specification and claims as xe2x80x9coxygen-selective ceramic membranesxe2x80x9d). For instance, J. D. Wright et al., xe2x80x9cAdvanced Oxygen Separation Membranesxe2x80x9d, pp 33-61 (1990) discloses an integration in which compressed air is indirectly heated to the requisite membrane operating temperature by a fired heater. The air is then passed through the retentate side of the separator where a portion of the contained oxygen is transferred to the permeate side by a pressure driven ion conducting ceramic membrane. The oxygen depleted retentate is heated in a fired heater to turbine inlet temperature and is then expanded in a turbine to produce power. The fired heater contains a heat exchange coil for heating the separator feed. A similar integration is shown in U.S. Pat. No. 5,516,359. In this patent, air is compressed to an elevated pressure and is heated to a membrane operating temperature by a burner or by indirect heat exchange. The heated compressed air is then introduced to the retentate side of a membrane separator that extracts oxygen from the air. The oxygen depleted retentate is further heated to a turbine inlet temperature by direct combustion before being expanded in a turbine to generate power. U.S. Pat. No. 5,562,754 discloses the introduction of steam into the oxygen depleted retentate stream as a replacement for the separated oxygen and also deploys steam as a sweep gas for the permeate side of the membrane to improve the driving force for oxygen transfer.
U.S. Pat. No. 5,852,925 describes different process options that are especially suited for retrofitting existing installations. In one option, only a portion of the compressed air stream is processed by the membrane separator. The resultant oxygen depleted retentate is combined with a stream that has bypassed the separator prior to turbine expansion. Another option provides a separate air compressor to supply the membrane separator. The oxygen depleted retentate is heated in a second stage combustor and is then expanded in a turbine.
U.S. Pat. No. 5,865,878 introduces various concepts of integrating an oxygen-selective ceramic membrane with a gas turbine in which such reactants as steam and natural gas are introduced into the permeate side of the membrane separator to react with the permeated oxygen to form desired products such as syngas.
U.S. Pat. No. 5,820,654 discloses a process and apparatus in which oxygen is extracted from a heated oxygen containing stream by an oxygen-selective ceramic membrane in which the oxygen product is cooled through indirect heat transfer with a portion of the incoming air stream. The gas separation and cooling are integrated within a single apparatus to maximize the use of conventional materials and construction.
All of the foregoing references disclose separator-gas turbine integrations that require the use of ancillary equipment such as heat exchangers and long piping systems for extracting air and re-injecting oxygen depleted air. As may be appreciated, such equipment and piping adds to the complexity and expense of the integration of membrane separator and gas turbine. Additionally, long piping runs produce pressure drops and difficulties in providing the separator with a uniform flow distribution.
As will be discussed, the present invention provides oxygen separators and methods, employing oxygen-selective ceramic membranes, that are designed for integration with a gas turbine without the use of long piping runs. As a result, the pressure drop involved in handling the large air flow between the components of the system is minimized and flow distribution problems are reduced.
In one aspect, the present invention provides an oxygen separator for separating oxygen from a heated oxygen containing gas discharged from an expander of a gas turbine hot gas generator used to drive a power turbine. It is to be noted that a hot gas generator consists of an air compressor, a combustor and an expander which drives the compressor. The expander exhaust is at both elevated pressure and temperature and can be used to drive the power turbine which normally is on a separate shaft from the compressor-expander shaft. Usually the hot gas generator-power turbine combination is an aircraft derivative design.
The oxygen separator utilizes a duct open at opposite ends and configured to be directly mounted between the expander of the hot gas generator and the power turbine in an in-line relationship to receive the heated oxygen containing gas from the expander and to discharge an oxygen depleted gas to the power turbine. A plurality of oxygen-selective ceramic membranes are provided for extracting oxygen from the heated gas. Such membranes are mounted within the duct so that the oxygen separates from the heated oxygen containing gas. The separated oxygen collects within the oxygen-selective ceramic membranes and an external flow of the oxygen depleted gas forms within the duct. A means is provided for recovering the oxygen from said oxygen-selective ceramic membranes.
Since a duct containing the oxygen-selective ceramic membranes directly connects the exhaust of the expander with the power turbine, the integration is simply accomplished and with the avoidance of a significant pressure drop in extraction of the heated oxygen containing gas from the expander and the reintroduction of the oxygen depleted gas to the power turbine. Further, where oxygen separators are not integrated in the manner set forth above, pressure drops as high as between about 3.45 bar and about an 5.52 bar are often required at the reintroduction point to achieve adequate distribution. This is inefficient in that it requires a greater degree of compression in the first instance.
Another integration is with the burners of a gas turbine of an industrial type. The turboexpander of these units drives both the air compressor and other connected load such as generators or process compressors. The exhaust from the tuboexpander is typically at near atmospheric pressure. This aspect of the present invention provides an oxygen separator for separating oxygen from compressed air flowing to a burner of a gas turbine. An elongated duct, open at opposite ends, is configured to be connected to the burner of the gas turbine to receive a heated oxygen containing gas formed from the compressed air after having been heated and to discharge an oxygen depleted gas. A plurality of oxygen-selective ceramic membranes are provided for extracting oxygen from the heated gas. Such membranes are mounted within the duct so that the oxygen separates from the heated oxygen containing gas. The separated oxygen collects within the oxygen-selective ceramic membranes and an external flow of the oxygen depleted gas forms within the duct. A means is provided for recovering the oxygen from the oxygen-selective ceramic membranes.
The duct can be mounted between the burner and the gas turbine. Alternatively, a pre-burner can be provided to heat the compressed air and the duct is directly mounted between the pre-burner and the burner of the gas turbine. In such embodiment, the duct can form an inner duct. An outer duct, surrounding the inner duct and connected to said pre-burner, defines an annular space between the inner and outer ducts to transfer the compressed air to the pre-burner.
Such integration in accordance with the present invention is particularly advantageous in instances where there are overriding space constraints for the installation of the oxygen separator. Additionally, it allows for a simple integration in which a good distribution of the fuel-retentate mixture to the burners of the gas turbine is assured with substantially less required pressure drop than is required in separate systems of the prior art.
In either type of integration, the oxygen-selective ceramic membranes can be in line with the flow of the heated oxygen containing gas or at an angle thereto, for instance, at right angles. Additionally, each of the oxygen-selective ceramic membranes can be of elongated, tubular configuration and have closed ends and opposite, open ends. In such embodiment, the recovery means recover the oxygen from the open ends of the oxygen-selective ceramic membranes.
Advantageously, in an oxygen separator using tubular ceramic membranes, a plurality of elongated tubes can be coaxially located within the oxygen-selective ceramic membranes for injection of steam to purge the oxygen from within the membrane. A steam plenum is in communication with the elongated tubes and a steam inlet line passes through said duct and is connected to the steam plenum for introduction of the steam into the oxygen-selective ceramic membranes. This purge helps drive the oxygen permeation through the membrane.
Another advantageous alternative feature of an oxygen separator of the present invention using ceramic membranes of tubular form is to provide a shroud surrounding the oxygen-selective ceramic membranes. A supplemental cool air inlet passes through the duct and is connected to the shroud for introduction of cooling air to cool the oxygen product within the oxygen-selective ceramic membranes and the structure supporting the oxygen-selective ceramic membranes. In such alternative, a plurality of tube-like sleeves can be mounted within the oxygen-selective ceramic membranes to create a narrow flow annulus and thereby improve a heat transfer film coefficients on the side where the oxygen is flowing within the oxygen-selective ceramic membranes. The advantage of the forgoing feature of the present invention is that it allows for a cooler operation of the oxygen-selective ceramic membranes in regions where such membranes are to be sealed and supported and therefore, the use of conventional construction and materials.
In a further aspect, a plurality of burner tubes are provided that are fabricated from oxygen-selective ceramic membrane material for separation of the oxygen from the heated gas. A fuel inlet line passes through the duct for introduction of fuel and a fuel chamber is provided in communication with the open ends of the burner tubes. The fuel chamber is connected to the fuel inlet line to introduce the fuel into the burner tubes for combustion of the fuel in the presence of the permeated oxygen. The combustion produces combustion products including carbon dioxide. Transfer tubes are coaxially located within tubular oxygen-selective ceramic membranes and the burner tubes for transfer of the combustion products from the burner tubes to the oxygen-selective ceramic membranes.
In either type of integration contemplated by the present invention, oxygen-selective ceramic membranes of tubular configuration can be mounted within the duct through connection to a tube sheet that is itself connected to the duct. The oxygen recovery means can be formed of a header plate connected to the tube sheet and having at least one opening to allow passage of the oxygen from the open ends of said oxygen-selective ceramic membranes through the header plate. Additionally, a cover is connected to said header plate that covers the at least one opening and a discharge line is connected to the cover and passes through the duct.
In a still further aspect of the present invention, the duct can be formed by at least two sections with the header plate connected to at least one of the two sections and the tube sheet connected to the other of the two sections. In such aspect, the header plate and said tube sheet can be provided with peripheral flanges connected to one another to connect said two sections to one another.
Either type of integration contemplated by the present invention can also employ a supplemental cool air inlet of the duct for introduction of cooling air into the duct to cool the oxygen within the oxygen-selective ceramic membranes while heating the air. For such purposes, at least one opening of said header plate can comprise aligned radial arrays of openings. The tube sheet, header plate, and cover each can have an annular configuration to define aligned, concentric central inner openings thereof. A sleeve can be connected to the tube sheet and aligned with the central, inner opening thereof to conduct the heated oxygen containing gas to oxygen-selective ceramic membranes of tubular form downstream of the supplemental cool air inlet of said duct.
In yet another aspect, the present invention provides a method of separating oxygen from a heated oxygen containing gas discharged from an exhaust of a gas turbine hot gas generator used to drive a power turbine. In accordance with such method, the heated oxygen containing gas is received at one end of a duct mounted directly between the expander of the hot gas generator and the power turbine in an in-line relationship. The oxygen is extracted from the heated oxygen containing gas by permeating ions of the oxygen through a plurality of oxygen-selective ceramic membranes. The oxygen-selective ceramic membranes are mounted within the duct so that the oxygen separates from the heated oxygen containing gas by permeation. The permeated oxygen collects within the oxygen-selective ceramic membranes and an external flow of the oxygen depleted gas forms within the duct. The oxygen depleted gas is discharged from an opposite end of the duct to the power turbine and the oxygen is recovered from the oxygen-selective ceramic membranes.
In another aspect, the present invention provides a method of separating oxygen from compressed air flowing to a burner of a gas turbine. In accordance with such method, the compressed air is heated by burning a fuel to form a heated oxygen containing gas. The heated oxygen containing gas is received within an elongated duct open at opposite ends and connected to the burner of the gas turbine. The oxygen is extracted from the heated oxygen containing gas by permeating ions of the oxygen through a plurality of oxygen-selective ceramic membranes having closed ends and opposite, open ends. The oxygen-selective ceramic membranes are mounted within the duct so that the oxygen separates from the heated oxygen containing gas. The separated oxygen collects within the oxygen-selective ceramic membranes and an external flow of the oxygen depleted gas forms within the duct. The oxygen depleted gas is discharged from an opposite end of the duct and the oxygen is recovered from the oxygen-selective ceramic membranes.
In accordance with the directly foregoing aspect of the present invention, the heated oxygen containing gas can be received within one of the opposite ends of the duct from a pre-burner connected thereto and the oxygen depleted gas can be directly discharged to the burner of the gas turbine from the other of the opposite ends thereof.
An alternative inventive aspect of the foregoing method is to utilize ceramic membranes of tubular configuration and to introduce fuel into burner tubes mounted within the duct and fabricated from oxygen-selective ceramic membrane material for separation of the oxygen from the heated gas. The fuel is burned in the presence of oxygen permeated through the burner tubes to heat the compressed air stream and to form combustion products including carbon dioxide. The combustion products are transferred from the burner tubes to the oxygen-selective ceramic membranes to purge the oxygen.
In either method of the present invention, the oxygen after having been recovered is cooled and then compressed. The oxygen-selective ceramic membranes can be purged with an inert purge gas, preferentially steam, which can be separated from the oxygen simply by condensation. Further, a supplementary compressed feed air stream at least equivalent in volume to the oxygen product removed can be compressed and introduced into the duct to cool the oxygen and supporting structure.